Funds are requested so that two of the X-ray diffractometers used for macromolecular crystallography by investigators in the Yale Faculty of Arts and Sciences can be upgraded. The upgrade has two components: (1) replacement of a Rigaku RU300 rotating anode generator that currently supplies X-rays to both an R-Axis IV and a MAR 345 diffractometer, and (2) replacement of the associated X-ray optics. The RU300 generator in question, which was purchased in 1986, is increasingly prone to failure due to age, and even when operating properly, the X-ray beams it produces are not well matched to the crystallographic problems now being pursued, which involve smaller crystals than those usually studied in the 1980s, when the RU300 was purchased, and many of these crystals have large unit cell dimensions, which makes data collection even more challenging. The plan is to replace this generator with a Rigaku MicroMax-007 rotating anode generator. Not only should this generator be more reliable than the RU300 because it is newer, but because it is a microfocus generator, everything else being equal, its smaller, brighter beam should also enable investigators to collect diffraction data from almost all the crystals they are now studying much faster and more accurately than is now possible. In order to take full advantage of this MicroMax-007 generator the Osmic monochromators now mounted on the RU300 generator, which were built for generators that produce large beams, will have to be replaced with the VariMax optical components Rigaku recommends. Funds are requested for two such optical systems, so that both of the area detectors currently installed on the RU300 generator can be mounted on the new generator. It is anticipated that this upgrade will significantly increase the rate at which structures are determined at Yale.